1. Technical Field
The present disclosure refers to a method for an improved checking of repeatability and reproducibility of a measuring chain, wherein testing steps are foreseen for multiple and different devices to be subjected to measurement or control through a measuring system comprising at least one concatenation of measuring units between a testing apparatus (ATE) and each device to be subjected to measurement or control.
The disclosure in particular, but not exclusively, relates to a methodology foreseen for quality control through the semiconductor device testing, and the following description is made with reference to this field of application with the sole purpose of simplifying its presentation.
2. Description of the Related Art
As it is well known, R&R (Repeatability and Reproducibility) methodologies, applied to measuring systems, and in particular to quality control through testing of semiconductors, have the purpose of ensuring the repeatability and reproducibility of measuring.
Currently, such R&R methodologies are strongly linked to each individual product that is also part of a plurality of products to be subjected to testing with R&R methodology.
In order to try to better understand the problems of this field of application it is worth noting that a generic measuring system is formed from a measurement chain MC arranged in cascade with each other and that represent a concatenation of test units TU, as schematically illustrated in FIG. 1.
For the sake of simplicity of presentation we can consider that each measuring system can actually be represented with a chain of basic test units arranged in cascade, but in any case this is not limiting for the purposes of the present disclosure.
Each test unit TU that forms part of the measurement chain MC can also have a complicated internal structure and interfaces with the other units exchanging information in the form of input/output signals that can be electrical, electronic, luminous or in the form of electromagnetic waves or radiation. The ways in which the information is exchanged between the various units TU is also not limiting for the purposes of the present disclosure.
In general, upstream of the systems chain there are measuring and stimulation instruments, represented here by a single overall system or basic unit called ATE (Automatic Test Equipment).
The ATE is formed from at least one resource capable of performing at least one type of measurement.
Each resource of the ATE in general should respect a specification. In its most common formulation the specification foresees that the resource can measure values of a specific magnitude located between a minimum LSL (Lower Specification Limit) and a maximum USL (Upper Specification Limit), as schematically illustrated in FIG. 2.
If the measurements are located within the specification range [LSL; USL] it is considered that the considered resource is working correctly (Pass) within its specification. Otherwise, if the measurement does not satisfy the specification, it means that such a resource has an anomalous behaviour (Fail) that can be investigated and evaluated in order to restore the correct operation of the resource itself.
In order to check the specification values of the various resources of the ATE, a calibration tool can be available that, hereafter, we shall call Checker, which can be of the hardware (HW) and/or software (SW) type and that is used following a dedicated procedure.
This calibration tool just checks that the various resources are within the specification and indicates as possibly anomalous only those resources that are not within their specification and that give a “Fail” result.
Downstream of the measurement chain MC there is the object or device that should be subjected to measurement, represented by a single overall unit called DUT (Device Under Test) schematically illustrated in FIGS. 1 and 3.
Of course, the DUT can consist of at least one element on which it is desired to perform at least one measurement, to obtain at least one parameter that is measured with at least one measuring technique, or the value of which is estimated from at least one measurement.
In general, it is also possible to perform the measurements of a plurality of DUTs in parallel, as shown in FIG. 3, obviously if the measuring chain has this ability.
However, for the purposes of the present disclosure, it should be considered that all of the DUTs measured in parallel are like a single overall DUT.
Between the ATE and the DUT there can be at least one test unit TU of the systems chain SC that acts to adapt and/or interface together the ATE and the DUT. This unit can also expand the measuring potentials and capabilities of the ATE, or focus/size the capabilities of the ATE to the specific DUT.
It may also be the case that, even if the resources of the ATE respect their specification, this may not be sufficient to satisfy the quality specifications for the testing of the DUT.
Indeed, the last purpose of the measuring system as a whole is that of identifying the DUTs that are defective, performing at least one measurement on the DUT itself and determining whether or not the DUT is defective based upon some criteria.
After the measurement, the defective DUTs can be separated from the others and discarded.
In the case in which an ATE incorporates at least two different resources of the same type, even if such resources satisfy their specifications, it can be foreseen for such resources to provide even considerably different measurements of the same magnitude measured whilst operating on the same DUT.
In short, the testing system should overall satisfy not only its own specification, but repeatability and reproducibility of the measurement should also be taken into account. In this field by repeatability we mean the variation of the measurement obtained with a single measuring resource ATE, which at different times carries out the measurement of the same characteristic on the same DUTs; therefore, the measurement variation on the same ATE is measured.
By reproducibility we mean the variation in the average of the measurements made by different ATEs that measure the same characteristic on the same DUTs.
The R&R technique in itself combines repeatability and reproducibility and it is calculated using ANOVA (ANalysis Of VAriance) statistical techniques, as schematically illustrated in FIGS. 4A-4C, where the word Appraiser indicates the measuring chain MC.
According to the current prior art technique, the so-called golden samples, or reference units, are used, which are inserted into the measurement chain MC in place of the DUT, on which the various ATEs (each with its own system chain) perform the measurement of at least one characteristic for N (N>1) times that we shall call Cycles, in M (M>1) different time periods that we shall call Runs, on the different measuring systems chains, as schematically illustrated in FIG. 5.
These golden samples are generally linked to the specifications for the end product, and usually are samples of the same end product.
In particular, in the technical field of semiconductor devices, if the DUT consists of chips on a wafer, the measuring chain MC generally comprises the ATE that is interfaced with the DUT through test units TU referred to as a Jig unit (or a DIB-Device Interface Board) and a probe card, as illustrated in FIG. 6A.
The Jig unit is optional and might not be present in the measuring chain. The Jig unit comprises circuits that interface the ATE with the device DUT through the probe card; the latter comprises different probes that make contact with the terminal pads of at least one chip on the wafer.
If the device DUT comprises at least one chip encapsulated in a package, or more than one chip encapsulated in the same package (System in a Package), the measurement chain MC comprises the ATE that is interfaced with the DUT through a Jig unit (or DIB) and a board with a test unit TU known as a socket in which the package that contains the chip or chips is housed.
Therefore, the golden samples can be some chips present on the wafer or in a package, in various forms, and quality control foresees that the R&R, in other words the Repeatability and Reproducibility, of the entire measuring chain be evaluated.
The evaluation of the R&R is carried out using an assortment of measuring chains MC, i.e. measuring chains with various combinations of one or more ATE devices with various adapters and interfaces as the test units. The evaluation is therefore carried out on the various measuring chains using the golden samples, performing the measurement of at least one feature or characteristic parameter of such golden samples.
In general, it is suitable to measure a few tens of different characteristic parameters, which are monitored with suitable statistical techniques, like for example the methodology known as “ANOVA”, to ensure the R&R, i.e. the Repeatability and Reproducibility of the various measuring systems.
Based upon the various and different types of DUT, the family and/or configuration type of the ATE and the consequent measuring chain MC shall of course change. In any case, checking the R&R of measuring systems in normal working conditions is generally a quality specification that should be satisfied, and it should be considered ever more important the higher the desired quality levels, for example for products in the automotive sector.
Given that the golden samples are linked to each product, in the cases in which there are many different products, the repeatability and reproducibility of the measuring systems should be checked on different products, which is a great burden because of the large number of golden samples to be identified and certificated.
Moreover, for each group of golden samples it may be desired to provide a specific setup of the measuring chain, to obtain the data from which to calculate repeatability and reproducibility.
Consequently, it becomes a great burden both in terms of time and costs and in terms of resources, to check repeatability and reproducibility on the different measuring chains, using the various types of golden samples.
If then the measuring chains are used in a production context, part of the time of the various measuring chains to carrying out measurements on the golden samples is dedicated to check repeatability and reproducibility.
Moreover, there will also be a plurality of types and families of golden samples that will not always be the same, since for each new product new golden samples, that add to the existing ones, should be identified and certificated.
In performing the check of repeatability and reproducibility the golden samples can become damaged and therefore new golden samples are introduced, and this has an impact upon costs, time and resources. In particular, in the field of semiconductors, the golden samples are frequently damaged and would often be replaced.
The greater the human involvement to carry out the measurements, the lower the repeatability and reproducibility of the measuring system, and this can often be partially solved with the introduction of automation of the measuring process.
Considering various measuring chains, if one of them is not repeatable and reproducible with respect to the others, then it will not be known which part of the measuring chain does not satisfy repeatability and reproducibility, and therefore it will not be known a priori on which elements of the measuring chain there needs to be an intervention to restore repeatability and reproducibility of the entire measuring chain to thus align it with the other measuring chains.
For example, if the golden samples are chips still on a wafer, they are interfaced with the measuring chain through a probe card the probes of which damage the contact terminal pads. In general, the contact between probe and pad is not repeatable and reproducible, since the probe that makes contact with the pad changes and damages the pad itself, and this can bring problems in the electrical measurements carried out by the measuring chain on the device of the wafer.
Particles can build up or can form oxides on the probes as well that alter the measurements and that decrease the reliability of the evaluation of repeatability and reproducibility of the overall system.
Indeed, all of this can for example translate into a random variation in contact resistance (in general impedance) between probe and pad, which alters the value of the measurements.
The probe card is subject to wear and deterioration, and this influences the measurements taken for repeatability and reproducibility; indeed, the probes wear out over time and change their mechanical parameters, like: the alignment of the probes, their planarity, their length, the force exerted on the pad, the shape of the tip.
This list is not exhaustive of all of the possible deterioration that can occur on a probe.
Over time, by using the probes the measurement can also be repeated, but the same measurement conditions cannot be reproduced.
Furthermore, after a large number of contacts that damage the pads, some wafers can no longer continue in the production process and therefore are not sold, but must be discarded with a consequent additional cost.
By using production devices like golden samples, if the normal testing methods of semiconductors are used, the various devices can be subjected to electrical and environmental stress that can alter their characteristics, and this reduces the reproducibility of the conditions in which the measurement is carried out.
For example, electrical stress can be used on the chips to check the oxide status, but such electrical stress can alter the oxide at crystal lattice level.
Consequently, both the action of the probes, and the action of stress ensure that it is not possible to carry out in full the repeatability and reproducibility check of the measuring chain on the DUTs that will then be measured in real conditions.
For example, in order to eliminate the effect of stress it is possible to avoid applying such stress during the repeatability and reproducibility check, and in order to eliminate the effect of the probes it is possible to use encapsulated chips as golden samples. However, by using encapsulated chips some interfaces should be introduced in the measuring chain that are different from those used for the measurements on a wafer, and therefore the measuring chain on which the repeatability and reproducibility check will be carried out should be changed in the different conditions in which the measuring chain itself will operate.
The measuring chains that have ATE with high parallelism are able to measure even hundreds of chips simultaneously, and therefore it is very complex, if not impossible, to use the golden samples approach.
Similarly, it can be said for measuring chains on which repeatability and reproducibility are carried out with golden samples of encapsulated chips that are the normal DUTs the chains should measure.
Indeed, even in these cases there will be contact between the package and the interfaces with the ATE.
It may be considered to solve to problem by welding the encapsulated chip onto a board, but this eliminates the presence of the socket, in which the encapsulated chips are housed to carry out the measurements in normal conditions.
Often for the measurement of the DUT just a part of the potential of the measuring chain is used and therefore also with golden samples only a part of the functionalities of the chain shall be verified.
Basically, if the measuring chain, in particular the ATE, has parts or resources that do not satisfy repeatability and reproducibility, this will be visible only with those types and families of golden samples that use this type of parts or resources to carry out the measurements.
Consequently, a measuring chain can be repeatable and reproducible for a certain class or family or type of golden samples but not for other classes, families or types.
Therefore, certain measuring chains can be used for the measurements of certain classes, families or types of DUTs and not for others, creating economic, managing and production problems, etc.
Moreover, the various golden samples use parts of the chain that are common to many types and/or families of golden samples and this is inefficient.
Therefore, by using golden samples linked to the end product, when considering a plurality of possible end products that are different to each other, it is difficult to ensure repeatability and reproducibility of the various measuring chains as a whole, and in the case of problems it is not easy to understand the causes that alter repeatability and reproducibility and to put it right.
All of this makes the current check of repeatability and reproducibility ineffective or inefficient, especially in a production context, also because the parameters measured are not many in general.